Other Parts Discussed in Thread: TXS0108E, , SN74AXC4T774
Hello:
We have an application in which we need to interface an MCU running at 1.8V to an external board that has an SD Card at 3.3V. We are using SPI to communicate to the SD Card at very low frequencies (100kHz initialization, 6 MHz running). We were reviewing the TXS0206A and the TXS0108E for this application, as described in the diagram below:
We were using their respective dev boards to test this idea. Both circuits are connected directly to the SD card with no pullups of any sort using these SD cards and jumper wire length of ~5cm.
However, we are running into issues when the SD Card is being initialized at 100 kHz. I can see with a logic analyzer on both sides that the MCU is sending the correct initialization routine. However for some reason the SD card is replying incorrectly when the TX0206A or TX0108e is in between the MCU and the SD Card (for example, to CMD0, it replies with a 0x07, 0x03, and rarely with a 0x01. I also tested that the system works when I connect the SD card directly to the MCU and power the MCU @ 3.3V. This way I can know the logic is ok).
In another test, I powered the MCU and the SD Card at 3.3V and put both devices in between, to make sure that it was not the 1.8V having issues. I had the same error result.
I have checked with the oscilloscope and cannot find any noise on both sides. However I do notice that sometimes there is a small glitch (<20ns) on the Chip Select line. As of now this is my only lead into what can cause the device to not function correctly.
Here is a capture from the logic analyzer on a failed initialization. The direct connection between an MCU@3.3V and the SD card does not present this glitch.
Searching the forum it seems this post was presenting something similar: e2e.ti.com/.../877297
I wanted to ask, what could I check to find the root cause?
Also, would there be a better circuit for this application? Would it be better to use another IC, or another configuration?
Thank you!